The downside of moving funds from an IRA to a 401k is that you are stuck with the investment choices and rules of the 401k with few options.

In my 401k (troweprice) I can trade in stocks along with ETFs, although I don't believe I can trade in options or short sales or penny stocks. But I haven't really partaken in those much. So not much of a downside.

Upside is the backdoor Roth, and potential to access funds freely at around the age of 55.